Does treatment of the pelvic nodes with IMRT increase late rectal toxicity over conformal prostate-only radiotherapy to 76 Gy?

Purpose: To compare late rectal toxicity rates after three-dimensional conformal radiotherapy to the prostate alone (P-3D-CRT) and whole-pelvis intensity-modulated radiotherapy along with a prostate boost (WP-IMRT/PB) to the same nominal total dose to the prostate. Patients and Methods: 68 patients treated with conformal radiotherapy to the prostate only to 76 Gy at the National Institute for Cancer Research, Genoa, Italy, represented the first group (P-3D-CRT). The second group consisted of 45 patients treated at the University of Texas Medical Branch (UTMB), Galveston, TX, USA, with IMRT covering the pelvic nodes and seminal vesicles to 54 Gy at 1.8 Gy per fraction and the prostate to 60 Gy in the same 30 fractions. A separate phase boosted the prostate to 76 Gy (WP-IMRT/PB). Major aspects of planning were remarkably similar at both institutions leaving the inclusion or not of pelvic nodes as the main treatment-related difference between the two groups. Late rectal toxicity was prospectively scored according to the RTOG scale. All patients have a 12-month minimum follow-up, and mean follow-up, similar in both groups, is 25.9 months (SD [standard deviation]: 8.4 months). Results: At 2 years, the estimated cumulative incidence of grade 2 late rectal toxicity is 6% ± 4% for WP-IMRT/PB and 21.2% ± 6% for P-3D-CRT (p = 0.06). The difference became significant (HR [hazard ratio] = 0.1, 95% CI [confidence interval]: 0.0-0.6; p = 0.01) at multivariate analysis. None of the patients developed grade 3+ toxicity. Conclusion: Despite the larger treated volume, WP-IMRT/PB allows more rectal sparing than P-3D-CRT.